Phosphole reagents

The phosphole reagents follow a more consistent trend for ( )-trisubstituted alkene formation (Table 20, entries 16,17,27,35,39,40,42,43) (42,147). The stereochemical preference is inverted by comparison to the conventional Ph3P=CHR reactions for the a-alkoxy ketones, for several enone examples (entries 16, 17, 27, 35), and for some of the a-alkyl branched ketones. This trend is similar to the E-selectivity pattern seen in the aldehyde reactions with phosphole-derived ylides, but more systems must be studied before reliable generalizations are possible. 

As underlined above, formation of these compounds by halogenation of dialkyl esters of 1,2-alkadienephosphonic acids is the major direction of the reaction. Even in the case of dialkyl esters of propadienephosphonic acid, some 1,2-oxaphosphole derivatives could be detected [86], On using sulfuryl chloride as electrophilic reagent, only 3,3-disubstituted- and 3-monosubstituted substrates could be transformed in oxaphosphole derivatives [39], Thus, the main role of the substituent at the C3 atom of the allenephosphonate system, for the formation of 2,5-dihydro-l,2-oxa-phosphole-2-oxide derivatives, was demonstrated. 

In 1989 Macomber and coworkers showed that the reaction of 1,2-alkadi-enephosphonic acids with different kinds of electrophilic reagents gave 1,2-oxa-phosphole derivatives (Scheme 17) [57],... 

When the trapping reagent is acetylene dicarbo-xylate itself, the reaction yields a new phosphorane which is a pentacoordinated phosphole (3). 

Notably, phospholes with controlled regioselectivity can also be obtained from zirconium complexes using silylated alkynes. For example, sequential treatment of Schwartz reagent 167 with 2-butyne, MeLi, and silylated alkynes... 

TT-complexes involving quaternized phospholes. Treatment of the dilithiophosphide reagent (193) with carbon dioxide in the presence of trimethylsilyl chloride results in the formation of the benzodiphosphole (194). A related reaction of the diphosphide (193) with pivaloyl chloride gives the benzodiphospholyl complex (195), which on subsequent treatment with alkyl halides is converted to the benzodiphospholes (196). A new route to the 1,3-diphospholide anion (197) has been developed, and applied in... 

The chemistry of the phosphole system, and that of related heterophos-pholes, has continued to be an active area. Treatment of unsymmetrical zirconacyclopentadiene reagents with phenyldichlorophosphine provides a route to the unsymmetrical phospholes (349). Organozirconium intermediates have also been used in routes to electropolymerisable heteroaryl-substituted phospholes, e.g. (350), and the bridged diphospholes (351). Routes... 

Electrophiles condense with the anion generated by treatment of the phosphole sulphide (101) with butyl-lithium either in the ring or at the methyl group depending upon the reagents used (Scheme 6). ... 

Using a procedure similar to that described here, or using isoiated zirconium metaiiacycies as reagents, we have been able to prepare not only phospholes, but also arsoles, stiboles, bismoles, siloles, germoles, stannoles, galloles, thiophenes, selenophenes, and borole Diels-Alder dimers. Since a number of other titanium and zirconium metaiiacycies are readily available, these reagents should be useful in the preparation of a variety of heterocycles. 

Reactions. - The reactions of diorganoiodophosphines with varying quantities of iodine have been studied by NMR techniques, which indicate the formation of a dialkyldi-iodophosphonium iodide as the initial product, followed by polyhalide anion formation in the presence of excess iodine. A modified McCormack synthesis of 3-silylated phosphol-3-enes (168) from phenyl-dichlorophosphine and 2-silylated 1,3-butadienes has been described. The reaction is carried out in the presence of a small amount of copper stearate in acetic anhydride at 50 °C, these conditions preventing the complete desilylation observed under standard conditions. The consecutive reaction of lithium phenylacetylide with triphenylborane and diphenylchlorophosphine results in the formation of the intramolecularly coordinated phosphino-borane (169). Treatment of the alkoxyvinyldichlorophosphines (170) with Grignard reagents... 

It has been found that nonstabilized ylides derived from the tetrahydro-phosphole nucleus (90 or 91) afford oxaphosphetanes that decompose at room temperature. Sin( 89, the phosphonium salt precursor of 90, contains only one alkyl group, BTP ylide 90 can be recommended for E-selective alkene synthesis in cases where the alkyl substituent must be used efficiently. Since the phosphorus environment in 90 is relatively expensive, this family of reagents will not provide a practical solution for large-scale synthesis of... 

Attempts to trap the less stable phosphole oxides by having ethyne derivatives present in the medium of their generation have not been successful, since the ethynes used so far have been sensitive to the reagents in the medium. 

The sensitivity of S to electrophilic reagents proved to be a complication when an attempt was made to perform cycloaddition with the oxyallyl cation, which had worked well with phosphole oxides. The reagent attacked on S, presumably to give a spirophosphorane, such as (177), which rearranged to leave oxygen on phosphorus as in (178) (Scheme 38) (31% <75T53 . Other products... 

In addition to the simple removal of sulfur to generate the phosphole (BujP or Zn/Hg), reaction at sulfur occurs with other reagents, and of special value is that taking place with certain coordination complexes. With Fe2(CO)9, phosphole sulfide (161) underwent a reduction-complexation process that resulted in the formation of the . With Mn2(CO)io, phosphole sulfide (161) underwent both loss of sulfur and of phenyl, giving rise to the phosphacymantrene derivative (181) <79JOM(l65)i29>. 

The chemistry of the hydro derivatives of phospholes, with P in the three-coordinate state, is dominated by the very high sensitivity of the phosphorus atom to various oxidizing and electrophilic reagents that result in conversion to higher coordination states (usually C.N.-4). In this sense their chemistry is not exceptionally different from common tertiary phosphines. The attack of electrophiles normally occurs first at phosphorus, and the product may react further at the double bond. 

The phenyl group on phosphorus can be replaced by an alkyl group on reaction with alkyllithium reagents <72T417l>. This has proved to be a process of considerable value for the introduction of the t-butyl group on phosphorus 3,4-dimethyl-l-/-butylphosphole was prepared for the first time by this reaction. The reaction is conducted in hexane solution at room temperature in the presence of TMEDA it is presumed to proceed by expansion of the coordination number of phosphorus with formation of a phosphoranyl anion (233). This anion returns to the three-coordinate phosphole by loss of phenyl anion. The phenyl group may also be displaced by -butyl in similar fashion <720MR(4)i7i> (Scheme 59). 

The P-phenyl group of a phosphole can be directly displaced by reaction with alkyl lithium reagents in TMEDA. Both t-butyl <72T47i> and -butyl <720MR(4)171> have been placed on P by this method. With 3,4-dimethyl-1-phenylphosphole, the former reaction occurred in 70% yield, the latter in 31.5% (with some oxide) (Scheme 79). This method is of considerable value for the introduction of the t-butyl group on phosphorus, as this group cannot be used as the P-substituent in a phosphonous dihalide in the McCormack reaction because of steric restrictions. 

The complexes (CXVI) are analogous to the derivatives (CIV) discussed in Section V, B. The tetraphenyl derivative (CXVI. 1) gives the tetracyclone-Fe(CO)3 complex (CXIII.l) upon bromination and with methanolic NaOH yields tetraphenylbutadiene-Fe(CO)3 (38, 39, 128). Reaction of this tetraphenyl derivative with appropriate reagents has provided a novel synthesis of furans, thiophenes, selenophenes, pyrroles, phospholes, and arsoles (38, 78, 128). 

Phospholes are known to exhibit characteristic optical and electrochemical properties derived from the phosphorus-bridged 1,3-dienic jt system [34]. Particular interest has recently been paid to their Jt-conjugated derivatives, such as nonfused phospholes, dibenzo[f ,d] phospholes, benzo[f ] phospholes, benzo[c] phospholes, and related compounds [35]. There are several methods to synthesize phospholes [36]. The classical method for the synthesis of phospholes is the reaction between the nucleophilic substitution of a P-X bond with a stoichiometric amount of an organometallic species such as organolithium or organomagnesium reagents (Scheme 4.21) [37]. 

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